It is a very good read but basically it's a "program" you follow in order to get all your "todo" lists into one place and how to manage your tasks.
I actually followed the program and was going along swimmingly for a couple of weeks, but if you don't follow it very closely it's easy to slip back into old habits. Even now I am in a state that I am not 100% following it, I am still better off than not following it at all.

If you don't feel like reading the whole book I will try to summarize in my words what I got out of it - what I say below may not be what the book was trying to get across, it's my interpretation. If you have read it and got something else out of it please add a comment so we can learn from each other.

Basically, if you have things you need to do "in your mind", you will constantly be thinking about them at times when you should be doing something else - they will drift in and out of your consciousness which increases your stress levels.

In order to feel at ease with all the tasks you need to do (no matter how many you have), they all need to be documented, not in your head. Until you have them on a list and all out of your head, they will keep popping back into your head and you will be stressing about them. If they are on a list and you can trust they are all on the list, then your mind can be free (at least for a while or at times - you can more easily be "mindful" in some situations when these extra thoughts are not lingering).

You should manage work and personal tasks the same way - they are all just things you need to do, so don't manage them differently (they can go on different lists, but how you manage those lists should be consistent).

When you have a "todo" list, you actually need to have a "next actions" list as well as a high level list of things to do. If you just have high level "tasks" to do, then you will keep skimming over them thinking "oh yeah, I will do that one day".. but if you actually write down a very specific "next action" such as "ring person x", then when you scan your list for things to knock off, it's much more achievable and you don't get distracted by the bigger more daunting "project". You still need the high level list of "projects", but you can just go through that maybe once a week and figure out for each one what the next action is if you don't already have one.

It is not feasible to work on tasks in "highest priority first" - it just doesn't work and I am sure everyone has found this out. There are 4 factors to consider when trying to determine what to do next:

Context/location

Time available

Energy available

Priority

So it makes sense to divide your "next action" list up into categories such as "At Work - On Computer", "At Work - No Computer", "At Home - On Computer", "At Work - No Computer". So the first thing you look at when determining something to do is "can I physically do this where I am, with the equipment I have?".

Next you see if you have enough time to do it, then whether you have the energy to do it and finally all other things being equal you look at the priority.

One thing I found that the book does not describe is that sometimes you need to plan to have the time and energy and be in the right place to get the high priority things done.. otherwise they never get done! (E.g. block out 2 hours in your outlook calendar to complete some task and let nothing distract you from that)

You need to have your "list"/"lists" with you at all times because as soon as you think of something you need to get it in there. When I started I was using Google Tasks. It is a feature within Gmail where you can track multiple lists of tasks, and tasks can have heirarchys. I had an app on my phone (GTasks) which sync'd with my Google Tasks list, so that if I was out and about, at work or at home I always had my list with me because I could access it from any PC or my phone.

After a while I moved over to Workflowy just due to my own personal preference using that one (I like categorizing using hash-tags because it makes it a bit more like a mind map than just a bunch of lists). Workflowy works on any internet connected device with a web browser so there is nothing to install and is free up to a certain amount of data/usage.

I hope that helps give you some tips even if you don't read the book.. or like me read the book but don't quite follow the rigid process!